With filtration according to the aforementioned applications, one uses a stationary filter disposed in a pressure vessel, wherein after completion of the filtration, a reset operation is carried out, comprising back-flushing of the filter before the start of the next filtration. The filtration takes place through at least one substantially vertically disposed filter element covered with a filter mesh surrounding at least one filter duct, through which flows the filtrate obtained during the filtration, e.g., white liquor or green liquor, while the filtered material, for example lime sludge (calcium carbonate) is deposited on the filter mesh. The pressure vessel is provided with at least one inlet for the suspension that is to be filtered, an outlet for the filtrate obtained, an outlet for the obtained material and an inlet for the medium for resetting the filter.
According to the prior art, in the utilization of a stationary filter during the lye treatment, the separation of white liquor and lime sludge is carried out in a so-called tube filter, i.e., downwardly suspended perforated tubes covered with filter mesh, where the filter outlet is directed upward. This tube filter is divided into discrete units having separate white liquor outlets from each section. While sections of the tube filter are being back-flushed, which is done with liquid that has already been filtered, i.e., white liquor, the filtration takes place in the other sections. This results in, inter alia, a low rate of utilization of the filter. In addition, during the entire process there is a continuous production of deposited material, i.e., lime sludge, from the back-flushing. This entails the disadvantage, among others, of the lime sludge having a high content of unfiltered liquid with a high content of chemicals.
Swedish patent application no. 7909774 describes a pressure filter and a method of filtration wherein the filtration time is altered as a result of the filterability of the suspension that is introduced. This means that one will have no indications of the condition of the filter.